Among various wastes generated by industrial production, those containing harmful substances such as zinc, lead, and the like are particularly required to be treated safely.
As the zinc-containing waste, there are one such as pressed scraps of galvanized sheets containing the zinc in the metal state and the other such as steel dust containing the zinc in the oxide state.
In the case of the zinc-containing waste in the metal state, as disclosed in Japanese Laid-Open Patent Publication No. 4-346681, it is possible to recover the zinc from the waste by heating the waste under vacuum to vaporize the zinc from steel. Thus, metal zinc can be recovered comparatively easily.
In the case of the zinc oxide-containing waste which contains the zinc in the oxide state, it is difficult to recover the metal zinc by the above-described method.
Thus, in treating the waste containing the zinc oxide, the following method is known: The waste is heated to a temperature of 1,200.degree. C. or higher by means of a burner in a rotary kiln method so that the ZnO (zinc oxide) makes a reducing reaction with coke, coal, or the like.
However, since this method necessitates the waste to be heated to a very high temperature, it leads to a highly increased energy cost. Further, the method has another problem that the metal zinc obtained by reducing the zinc oxide is oxidized again when it is recovered and thus it is difficult to recover zinc in the metal state. Furthermore, the method is uneconomical unless the waste is treated in a large amount.
As another method, the zinc is vaporized by plasma at a high temperature to recover the zinc in the metal state by means of a lead splash condenser. However, it is difficult for this method to perform so-called on-site treatment that is capable of treating the whole amount of the generated waste at the site, such as a plant, where the waste has been generated, because this method exerts bad influence unavoidably on environment in treating the waste. Thus, it is necessary to transfer the waste to a specific waste-treating site having an environment-nonpolluting equipment.
Under these circumstances, the zinc is recovered in the zinc oxide state from a part of the waste containing the zinc oxide by a rotary kiln method and then the recovered zinc oxide is refined to recycle it, whereas the remaining part of the waste is transferred to the specific waste-treating site to have it reclaimed there, in consideration of the legal regulation.
However, it is difficult to continue to perform the above-mentioned controlled discard of the waste in the future due to problems such as a shortage of the waste-treating sites and a high processing cost.
Moreover, it is not economically preferable to discard materials such as the zinc contained in the waste without recycling them.
Under these circumstances, needs exist for the development of arts capable of recovering the zinc with other harmful substances from the waste containing the zinc oxide and recovering the zinc in the metal state at a high purity to recycle it.
Mr. Azakami et al. of Tohoku University proposed a two-stage vacuum-reducing method for recovering the metallic zinc from zinc oxide-containing waste (quoted from magazines of Japan Mining Industry Society, No. 1203 of volume 104, No. 1206 of volume 104 and No. 1209 of volume 104.) However, since CO gas is used to reduce iron oxide contained in the waste in the first stage of the two-stage vacuum-reducing method, and then the zinc is reduced by using the reduced iron obtained in the second stage, a reducing reaction that lasts for more than three hours is required. Thus, this method is not economical and difficult to industrialize accomplishment of the on-site treatment.
In view of the above-described conventional problems, it is accordingly an object of the present invention to provide a zinc oxide-containing waste-treating method capable of recovering metallic zinc therefrom at a high purity in a short time period and accomplishing the on-site treatment.